There always is a risk for a traveling vehicle to crash into a preceding vehicle if the preceding vehicle makes an unexpected deceleration or stop and/or if the vehicle operator of the ego vehicle fails to maintain a safe headway distance to the preceding vehicle. To reduce such a risk, it has been proposed to use a radar to detect the headway distance to a preceding vehicle and the relative speed between the preceding vehicle and the ego vehicle, and compute the time to collision (TTC) or the time required for the ego vehicle to reach the preceding vehicle when the current ego vehicle speed is maintained. If the TTC falls below a prescribed threshold value, an alarm may be issued to encourage the vehicle operator to reduce the speed. It is also possible to activate the brake of the ego vehicle upon detection of such an event or an event where the ego vehicle could collide with the preceding vehicle.
Typically, the TTC can be given by the following formula:TTC=Xr/Vr=(Xp−Xe)/(Vp−Ve)where Xr: relative distance between the ego vehicle and the preceding vehicleXp: position of the preceding vehicle along the roadXe: position of the ego vehicle along the roadVr: relative speed between the ego vehicle and the preceding vehicleVp: traveling speed of the preceding vehicleVe: traveling speed of the ego vehicle.
It is known that a serious accident often occurs when a preceding vehicle collides with another preceding vehicle traveling immediately ahead of the preceding vehicle for a certain reason, and the resulting sudden stopping or deceleration of the preceding vehicle may cause the ego vehicle to collide with the preceding vehicle. In such a situation, the preceding vehicle comes to a stop or decelerates so suddenly that it is extremely difficult for the ego vehicle to stop or decelerate quickly enough to avoid the collision with the preceding vehicle. Sometimes, this event involves a large number of vehicles resulting in a massive traffic accident called “pile-up.”
There have been proposals to monitor not only the state of the first preceding vehicle travelling immediately ahead of the ego vehicle but also the state of the second preceding vehicle travelling immediately ahead of the first preceding vehicle to allow the ego vehicle to predict a deceleration of the first preceding vehicle well in advance and give the ego vehicle an ample time for taking a preventive measure.
However, if the warning is issued in an excessively conservative manner, the frequent and often premature issuance of warning may be found annoying to the vehicle operator so that the vehicle operator may lose trust in the system and may even keep the system turned off. Therefore, it is highly desirable to issue a warning only when there is a high risk for the ego vehicle to collide with the preceding vehicle. This can be accomplished by taking into account various factors that are evaluated from the road conditions and traveling conditions in determining the threshold value for issuing a warning.